Green roofs can help to reduce both the pollution and surface run off entering the drainage system. In this way they are often, in dense urban areas, the only applicable source control mechanism in the Sustainable Drainage Systems (SUDS) management train.

A green roof replicates what the landscape provides in terms of allowing infiltration into the vegetation, substrates and engineered drainage layers.

• A green roof will typically intercept the first 5mm and more of rainfall providing interception storage, the amount of which will be dependent on the depth and type of substrate in the green roof system.
• In the summer a green roof can typically retain between 70% - 80% of the runoff
• In Germany between 40% - 100% of rainfall can be retained –dependent upon the season 75% of rain falling on extensive green roofs can be retained in the short term and up to 20% can be retained for up to 2 months
• As the rainfall events become longer or more intense, the positive effect of a green roof remains as there is still a significant reduction in peak runoff rates.
• This increase in the 'time of concentration' means that a green roof will be beneficial throughout a wide range of rainfall conditions
• The above benefits collectively mean that by incorporating a green roof into new development, there will be a reduction in the amount and cost of the overall drainage infrastructure required to serve that development.

The table below is an abridged table from the FLL.